The present invention generally relates to fabrication of semiconductor devices and more particularly to the method of forming a metal film by a CVD process while using a metal carbonyl compound as a source material.
CVD (chemical vapor deposition) is a fundamental and important film-formation process used in the production of various semiconductor devices. As the film formed by a CVD process can achieve excellent step coverage even when the film is deposited on a complex structure having a large aspect ratio, the CVD process is used extensively for forming insulation films, semiconductor films and metal films in the fabrication process of semiconductor devices or display devices such as a liquid display panel.
Particularly, the pyrolitic CVD process of a metal film conducted by using a metal carbonyl source material is important in relation to the technology of multilayer interconnection in view of the fact that the process can form a refractory metal film such as a W film on an insulation film such as an SiO2 film directly with low specific resistance.
Conventionally, CVD of a W film has been conducted by using a source material such as WF6 or WCl6 by causing a reduction reaction in such a W-containing source compound by using H2, SiH4 or NH3. According to such a process, however, it has been difficult to deposit a W film on an insulation film such as an SiO2 film.
On the other hand, there is proposed a technology capable of forming a W film on an SiO2 film by using W(CO)6 as a CVD source material. According to this proposal, a W film is formed by conducting a pyrolytic decomposition reaction of W(CO)5 as follows.W(CO)6→W+6CO.
Typically, the CVD process of a W film using a W(CO)6 source material is conducted under the pressure of about 7 Pa (0.5 Torr) or less while setting the substrate temperature to about 500° C. Under such a condition, the deposition of the W film occurs immediately with commencement of the process, even in the case the process is conducted on an SiO2 film. Thereby, a high-quality W film is obtained with high throughput.
Meanwhile, there is an increasing tendency, in recent ultrahigh-speed semiconductor devices, of using a so-called low-K dielectric film having a characteristically small dielectric constant as compared with an SiO2 film, for the interlayer insulation film of a multilayer interconnection structure.
Because such a low-K insulation film generally has a poor heat resistance, it is necessary to conduct a deposition process at the substrate temperature lower than 500° C., such as about 400° C. or less, in the case a W film is to be deposited on such a low-K dielectric insulation film by using the CVD process that uses such a metal carbonyl compound as the source material.
FIG. 1 shows the relationship between the duration of deposition (deposition time) and the thickness of the W film observed by the inventor of the present invention in the experimental investigation that constitutes the foundation of the present invention. In this investigation, the W film was deposited by using W(CO)6 as the source material at the substrate temperature of less than 500° C.
More specifically, the experiment of FIG. 1 was conducted under the process pressure of about 8 Pa (0.06 Torr) at the substrate temperature of 413° C. while supplying a W(CO)6 source gas from a source bottle holding therein the W(CO)6 source material at 25° C., by causing a bubbling in the source bottle by an Ar carrier gas with a flow rate of 50 SCCM.
Referring to FIG. 1, it can be seen that the deposition of the W film on the substrate does not occur immediately after the commencement of the deposition process. The deposition of the W film starts only after an incubation time of about 5 minutes has elapsed. After the incubation time, it can be seen that the thickness of the W film increases generally linearly with the deposition time.
Thus, the relationship of FIG. 1 indicates that it is possible to form a W film on an SiO2 film with high precision also at such a low temperature by controlling the duration of the deposition process. On the other hand, the existence of such an incubation time inevitably decreases the throughput of the W-film formation process. In the case of a single-wafer process, in which substrates of large diameter are processed one by one, in particular, there is caused a waiting time for each wafer in correspondence to the foregoing incubation time, and a serious degradation of process throughput is caused for the overall process of semiconductor device production.
It should be noted that this incubation time increases when the substrate temperature is decreased and can reach as much as 600 seconds or more.
In the case of forming a W film by the pyrolytic decomposition process of W(CO)6 under the process pressure of 0.1 Torr at the substrate temperature of 338° C. by setting the bubbler temperature to 30° C. and the flow rate of the gaseous source material containing W(CO)6 to 50 SCCM, it was confirmed by the inventor of the present invention that, there is caused an incubation time of 618 seconds, which is longer than 10 minutes. Once the foregoing incubation time has elapsed, there occurs the deposition of the W film with the rate of about 6.6 nm per minute.